The electrical conductivity of a liquid is an important analysis parameter of electrochemistry. Its measurement has a wide application in fields like the chemical industry, metallurgy, biology, medicine, grain testing, water conservancy, energy resources, and others. Conductivity measuring methods can be divided into two groups: contact-type and non-contact type.
A non-contact type measurement applies the principle of electromagnetic induction and is therefore also referred to as electromagnetic conductivity measuring method or inductive conductivity measuring method. As there is no contact between the conductive part of the measuring component and the measured liquid, sensors of this type possess the advantages of good solidity, corrosion resistance, non-polarization and long service life.
There has been a long history of development since the basic principle of electromagnetic measurement of the conductivity of a liquid was first invented and applied in practice. For example, the U.S. Pat. No. 2,542,057 to M. J. Relis opened the basic theory to the public in 1951. The sensor according to this reference employs a pair of coaxial magnetic rings, covered with corrosion-protective and electrically insulating material. The inner hole of the two magnetic rings allows the current path through the liquid. According to the electromagnetic induction principle, by supplying an alternating current to the excitation coil an alternating magnetic flux is generated in the magnetic ring carrying the excitation coil, which, in turn, generates an induction current through the loop in the measured liquid. The induction current generated in the loop represents the current loop which passes through both the excitation-side magnetic ring and the pickup-side magnetic ring. This current loop generates an AC magnetic flux in the magnetic ring, which generates in the induction coil an induced current, which in turn produces an induced electrical voltage at the induction coil. The induced current is related to the conductivity of the liquid. The induced current and the induced voltage of the induction coil (open-circuit voltage) is proportional to the current through the liquid. Thus, the conductivity of the liquid can be derived from the measurement of the induced current or the induced voltage. The conductivity G of the liquid can be calculated from the formula G=C/R, wherein C is the sensor cell constant and R is the equivalent resistance of the loop through the liquid.
In the past, the induced current or the induced voltage of the induction coil was measured by an electric bridge balance method. The bridge balance reduces the effect of stray magnetic or electric fields, which are related to an unwanted coupling between the coils. However this method has the disadvantages of low precision and a low level of automation. At present, due to the development of the modern electronic technologies, this method is rarely used.
To increase the sensitivity and the precision of the measurements and to reduce the magnetic properties of the coils, U.S. Pat. No. 5,455,513 A1 to Neil L. Brown, et al. proposes a system that uses a current-compensation method, also referred to as zero-current method. The induction current in the induction coil is balanced with the compensation current in the measuring device. This results in that the compensation current is subtracted from the induction current to produce a difference signal. This difference signal is processed by amplification, in-phase detection and integration to provide a direct current (DC), also known as continuous current or DC signal, which is proportional to the conductivity of the liquid.
For the generation of the compensation current a feedback circuit is introduced. Thereby the DC signal is processed by a switching multiplication and further amplifications to derive a phase shifted, square wave feedback signal, which is supplied via a feedback resistor to the induction coil. The circuit is configured to provide a negative feedback: in the case that the two currents in the above mentioned subtraction are not equal, the resulting difference signal causes the generation of a feedback action that will bring the circuit back to balance, which corresponds to a difference signal at zero value.
However, this method and the corresponding circuit is relatively complicated, because it has many complex and expensive components like a tuned filter amplifier, an in-phase detector, switching multiplier and numerous amplifiers. To change the measurement range, one normally has to change the parameters of all these components.
Another design of an input circuit is disclosed in U.S. Pat. No. 4,220,920 to Thomas A. O. Glass. In this case the induction coil is connected via a coupling capacitor to an operational amplifier, configured as a current-to-voltage converter. The capacitor separates the induction coil from the operational amplifier and serves to eliminate a DC offset. While this appears to be a simple solution, it also has some disadvantages: first, the alternating current could be attenuated to some extent by the DC-blocking capacitor; second, the size of this capacitor is large; and third, connecting the negative input of the operational amplifier and the induction coil via a DC-blocking capacitor can lead to oscillations. A circuit without DC-blocking capacitor is therefore preferable.
In the measurement of conductivity, it has also become more and more important to improve the reliability of the measuring process. If not investigated carefully, an open-circuit failure of the coils or cable wiring could easily be mistaken for a conductivity of zero (or a very low conductivity). U.S. Pat. No. 6,414,493 to Behzad Rezvani discloses a method wherein a single-turn coil is added to each of the two magnetic rings. A large resistor is arranged in series with each coil in order to get a certain bias. Under normal conditions, this bias can be corrected by calibration. But when open-circuit failure occurs at the coils or cables, the measuring circuit will exhibit a significant negative conductivity, which allows the open-circuit failure to be detected with the method. However, adding a coil to the ferrite rings makes the circuit more complicated.